1. Field of the Invention
The present invention relates to an image forming apparatus, and more specifically, an image forming apparatus provided with a sheet storing unit wherein sheets with an image are stacked and stored.
2. Description of Related Art
Regarding a tray wherein sheets ejected from an image forming apparatus such as a copying machine, a laser printer, etc. are stored, in order to store a volume of sheets thereon, a type of tray which is moved down by elevate means in accordance with the volume of sheets stacked thereon has been commercialized. The tray is moved down so that the space between a sheet receiving portion of the tray and a sheet ejection portion (a nipping portion formed of pairs of ejection rollers) can be fixed in spite of variety of the volume of sheets stacked on the tray, and alignment of sheets on the tray can be stable.
This type of sheet storing apparatus was already disclosed, for example, in Japanese Patent Laid Open Publication No. 61-295970. A sheet storing apparatus disclosed in the publication further has a shifting function for dividing sheets right and left.
In this type of sheet storing apparatus, a tray is first moved up to the top position when a copying operation is about to start, and if sheets remain on the tray at that time, the copying operation is kept stand by. However, such a thing often happens, for example, in a case of an interrupt copying operation, and the inhibition of the copying operation in this case is inconvenient. Also, when all or some of sheets stacked on the tray are taken away therefrom during a copying operation, the successive sheets will be received by the tray at a lower position than a proper position, therefore causing disorder of sheets on the tray.
Incidentally, recently, an image forming operation in an image forming apparatus is getting faster. If sheets are ejected from the image forming apparatus onto a tray at a high speed, the sheets will jump up and push other sheets stacked on the tray, therefore resulting in disorder of sheets on the tray. As a countermeasure, Japanese Patent Laid Open Publication No. 60-31463 suggests the speed of a transportation system be reduced immediately before each sheet is ejected onto the tray. In executing such a speed control, if the timing of the slow-down delays on account of a slip of the former sheet, etc., the interval between the former sheet and the latter sheet will be small, and there is a fear of a paper jam.